Pipeline must be installed under surface barriers such as highways, waterways, buildings and other surface obstructions without disturbing the surface. Typically this has been done through a process of horizontally boring beneath the surface barrier. The basic process includes opening a trench on both sides of the barrier. A boring apparatus is placed on one side of the barrier and a passageway or bore is formed under the barrier between the two open trenches. The bore is of sufficient size to allow a section of pipeline to be pushed lengthwise through the bore from one side of the highway to the other. The installed pipeline section is then welded into the pipeline and tested.
When rock or other hard materials are encountered in the boring operation, problems can arise which cause the installation to be difficult and expensive. For example, when installing a 36-inch, 40-inch, or even larger pipeline under a 300-foot wide interstate highway barrier, massive forces can be present during the process of drilling the bore for the pipeline section. When hard rock materials is encountered in the drilling process using conventional large-diameter boring apparatus and methods, it is difficult, if not impossible to form the bore along a desired path. For example, when hard rock is encountered in the drilling process, the boring head tends to corkscrew, bend, and deviate from a straight path. This causes installation of straight pipe to be difficult, if not impossible, especially the longer the underground bore. If the bore is not sufficiently straight, the pipeline section can become stuck. The stuck pipe must be cut, the bore filled up, and a new attempt made at drilling a bore and installing the pipeline section. These and other difficulties in installing large-diameter pipeline beneath a barrier cause the process to be difficult and expensive.
U.S. Pat. No. 5,314,267 issued on May 24, 1994 to Mark Osadchuk, which is incorporated herein by reference in its entirety, discloses a horizontal pipeline boring apparatus and method for installing a pipeline section under a surface barrier, such as a roadway or the like. According to that invention, a pilot bore is formed under the barrier. Next, a boring head, which is sometimes referred to in the art as a reamer or a hole opener, is used to enlarge the pilot bore. In addition, a guide is positioned on the advancing side of the boring head. The guide on the boring head is designed to engage the walls of the pilot bore and help steer the pipeline boring head during cutting along the path of the pilot bore. The pipeline section is advanced behind the boring head. Drilling liquids can be supplied to the boring operation through the pilot bore, and an auger in the pipeline section is used to help move drilling mud and cuttings away from the boring head through the pipeline section. While the apparatus and method disclosed in U.S. Pat. No. 5,314,267 controls the boring operation, improvements are desired for further control of the boring operation.
The need for improvements is particularly long-felt for large-diameter pipeline sections. The larger the diameter of the desired bore, the greater the twisting force that are created in the drilling operation. According to the laws of physics, torque is the product of the force and the perpendicular distance from the line of action of the force to the axis of rotation. The hardness of the rock, the advancing force on the boring head, and all else being equal, for any given radial distance from the axis of the boring operation, the resulting torque is a product of that radial distance. Thus, the larger the boring head, the greater the perpendicular distance from the line of action of the force to the axis of rotation. The torque is created at every point along the radial cutting swath of the boring operation, such that the integral summation of these torques increases the wider the cutting swath of the boring operation.
For example, in opening up an 9 inch pilot bore to 30 inches in a single drilling operation, the cutting swath is about radial 21 inches wide. Thus, a 30 inch diameter boring head working against hard rock in this 21 inch wide cutting swath toward the periphery of the boring head creates a substantial twisting force (torque) about the axis of the pilot bore. If attempting to open up a 9 inch pilot bore to 60 inches in a single drilling operation, the cutting swath would be about 51 inches wide, and the tremendously increased torques involved would usually make such a drilling operation impossible. Thus, it is usually not possible to enlarge the initial pilot bore to a very large diameter bore in a single drilling operation.
To install a 60-inch pipeline, for example, the relatively small pilot bore must usually be opened up to at least one intermediate diameter. If very hard rock is encountered, it may be necessary to use several stepwise drilling operations to open up the pilot bore to successively larger and larger diameter bores until the desired diameter is achieved. For example, the pilot bore may be first enlarged to 24 inches, then in a second drilling operation be enlarged to about 42 inches, and finally in a third drilling operation enlarged to 60 inches.
Despite enlarging the pilot bore in stepwise drilling operations, in opening up a 42 inch bore to 60 inches, for example, the 60 inch diameter boring head working against hard rock in the 18-inch cutting swath toward the periphery of the boring head creates tremendous twisting force about the axis of the pilot bore. Even if the guide in the pilot bore helps maintains the drilling operation in a substantially straight line, the tremendous twisting force causes the drilling operation to drill eccentrically of the central axis of the pilot bore. With each successive drilling operation to increase the bore size, the off-center drilling creates an increasingly misshapen bore, which tends to become increasingly triangular and can be loosely described as an "A" shaped. This then requires that a substantially larger bore must be formed to install the desired large pipeline, which costs time and money.
Furthermore, the twisting forces created in the drilling operation can be so large that the boring head becomes increasingly likely to completely twist off its drive shaft, also referred to as a drill pipe. If the boring head twists off the drill pipe, retrieving the boring head can be very time consuming and expensive, and the boring operation may have to be abandoned in favor of a new attempt.
Thus, there has been a long-felt need for an improved boring head and method that is more easily controlled to form a straight bore, that produces a more perfectly circular bore, and is much less likely to cause the boring head to twist off the drill pipe.